A number of different types of cellular devices exist, often referred to as mobile devices, which communicate through cellular networks such as GSM and 3G. Other countries and territories may use different systems. The commonest type of cellular device is a cellular telephone or mobile telephone, though other devices exist, which need not in fact be mobile—GSM and 3G networks can also be used in fixed installations.
Cellular telephones may be sold either with or without contracts for the supply of mobile telephony services. In either case, the supply of mobile telephony services is typically regulated with a subscriber identification module (SIM) card. If the mobile telephony services are supplied with the cellular telephone, as is common in the consumer market, the SIM card may be bundled with the cellular telephone; if not, the SIM card may be supplied separately after concluding an airtime agreement.
The SIM card is programmed with an international mobile subscriber identity (IMSI) which identifies the subscriber. The IMSI includes a number of digits, the first being three digits of the mobile country code, MCC, which indicates the country, followed by two or three digits indicating the individual mobile network, the mobile network code MNC. The rest of the IMSI includes the mobile subscriber identification number which identifies the individual subscriber of the mobile network. Thus, the SIM card is specific for a particular mobile network. For this reason, when a cellular telephone is manufactured for multiple countries and/or networks, the SIM card cannot be bundled with the cellular telephone until the destination country and network are known.